In modern communications systems, it is common to configure a communications link comprising a pair of digital signal processors separated by an analog channel. For example, in an optical communications network, an optical link may include digital signal processors in the transmitter and receiver, that exchange signals through an analog channel that includes analog driver and modulator circuits in the transmitter, detector circuits in the receiver, and one or more optical fiber spans extending between the transmitter and the receiver.
Typically, the analog channel is formed as a series of circuit blocks defining, for example, filters and amplifiers. In order to enable differing biasing requirements of each block, it is common to insert an AC-coupling capacitor between adjacent circuit blocks. The AC-coupling capacitor is beneficial in that it blocks direct current (DC) flow between adjacent circuit blocks, while permitting relatively unimpeded passage of Alternating Current (AC) signal components. However, a limitation of this technique is that the AC-coupling capacitors in the analog channel operate as a high-pass filter which suppresses the signal components within a notch centered at 0 Hz. For direct detection receivers, this notch will appear in the received signal spectrum, centered on 0 Hz. On the other hand, in a coherent receiver, the detected signal is down-converted to a desired band for digital processing, and the notch may be located at a frequency that is offset from 0 Hz by an amount that is dependent on the frequency difference between the transmitter and receiver oscillators. In the case of an optical communications system, the transmitter and receiver oscillators are provided by lasers that are prone to random and deterministic frequency excursions. This means that the frequency difference between the two lasers fluctuates in time, and so the location of the notch in the received signal spectrum will tend to vary randomly within a bounded region centered on 0 Hz. This moving notch in the received signal spectrum has the effect of introducing significant distortion into low frequency components of the received signal.
Known methods of addressing the baseband notch, and the consequent low-frequency distortion in the received signal focus on attempting to reduce the width of the notch, reducing the frequency difference between the transmitter and receiver oscillators, and reducing the frequency jitter of the transmitter and receiver oscillators. In practice, reducing the width of the notch may be accomplished by increasing the size of the AC coupling capacitors. While the capacitors themselves are inexpensive, their physical size interferes with efforts to reduce the footprint of the transmitter and the receiver, and so tends to increase the cost of packaging these components.
It is known to apply linear operations, such as precompensation or Wiener Filtering, at the transmitter and/or receiver to reduce the penalty due to a lack of gain or excess noise in a subset of the channel's spectrum.
It is known to apply coding to a stream of information bits so as to alter the spectral characteristics of the signal created by on-off keying with that bit stream. Examples include 8B10B, AMI, and duobinary. However, with modern complex constellations the relationship between the patterns in the bit stream and the spectrum after modulation by that bit stream is very intricate.
It is known to apply modulation methods, such as spread-spectrum or CDMA, that significantly broaden the bandwidth of the resulting signal and provide corresponding resilience to narrow-band degradations. Most optical communications applications cannot tolerate the costs from a large increase in the signal bandwidth.
It is known to apply modulation methods, such as OFDM or DMT, where the bit stream is partitioned into a large number of subsets that are modulated onto parallel carriers, and different constellations are used in different subsets depending upon the noise level therein. However, channel nonlinearities can severely degrade the performance of a signal comprising a large number of parallel modulations.
Techniques for minimizing the requirement for large AC coupling capacitors in a communications system remain highly desirable.